The Boston Customs House (now renovated to a hotel) has an observation deck open 6 days/week at 2PM, with you making a reservation 48 hours in advance.

From the website:

<https://www.bostoncentral.com/customs-house-boston-observation-deck-views>

"...Customs House Observation Deck Tours Timing is everything when visiting this venue, as the 26th-floor observation deck is only open once daily (at 2:00 pm 6 days a week with the exception of Fridays), and weather permitting, so its a bit of a roulette play. But well worth it if the stars align for you.

Registration is required 48 hours in advance by calling 617-310-6300 (x4 - front desk). No back packs are allowed, and there are now only 8 people allowed per reservation."